This invention relates to an axial thrust balancing device for pumps.
In a pump, water is introduced through a suction port by the rotation of impellers and discharged through a discharge port while the respective stages of impellers provide the water with energy to raise its pressure. It is known that an axial thrust is produced by the pressure of water discharged through the discharge port and this axial thrust causes the impellers to move toward the suction side. To cancel such axial thrust, it has been proposed to use an axial thrust balancing device in which a balancing disk is mounted on a shaft behind the last stage impeller, and a stationary wall, such as a casing, extends between the balancing disk and the last stage impeller in a manner so as to define two small gaps between the stationary wall and the opposed balancing disk. One gap is an annular one, and the other gap is an axial one. At one end of the annular gap, a high pressure chamber is formed behind the last stage impeller. An intermediate pressure chamber is formed between the other end of the annular gap and one end of the axial gap. At the other end of the axial gap, a low pressure chamber is formed behind the balancing disk. Thus, the major part of the water introduced through the suction port of the impeller by the operation of the pump, is delivered through the discharge casing, but a part of the water discharged from the last stage impeller is leaked through the pressure chambers and the gaps to the suction side of the pump or to the atmosphere.
Under this condition, as an axial thrust acting on the impellers displaces the shaft to reduce the thickness of the small axial gap, the resistance of flow offered by the gap increases and the flow rate of water flowing therethrough is reduced, so that the pressure in the intermediate pressure chamber rises to push the balancing disk back. More specifically, if the balancing disk is designed in a suitable manner, the axial thrust produced by the pressure difference across the balancing disk can cancel the axial thrust caused by the impellers, to stabilize the displacement of the pump shaft. Such displacement is fine in its magnitude. However, when this construction is applied to a vertical pump, the balancing disk is moved downwardly toward the stationary wall by the weight of the rotary parts during suspension of operation, thereby making it necessary to provide means for avoiding contact between the balancing disk and the stationary wall.